For preventing the decay of wood and for minimising the detrimental effects of micro organisms, it has been recently tried to find natural and environmentally-friendly wood preservatives, because many state of the art impregnants have been discovered to contain materials harmful to the nature and to human health. A material harmless to the environment and well applicable to the treatment of wood is crude tall oil, a by-product of the pulp industry, and products processed from it. Also the use of some other vegetable oils is possible, but crude tall oil has been discovered to be the most suitable for the use, inter alia, because it is widely available and its costs are relatively low. Furthermore, studies have recently shown that, because of resin and fatty acids it contains, wood can be effectively preserved by crude tall oil from the wood-decaying effects of rot fungi and other microbes when a sufficient amount of crude tall oil is obtained in the inner parts of wood. With Finnish pine, for instance, the preservation effect has been observed with over 100 kg residues per one cubic meter of wood. This amount corresponds about 20% of the dry weight of pine. The amount of crude tall oil must be about double compared with e.g. creosote oil in order to achieve a sufficient preservation effect against decay. Until 2005, the NTR-A class retention requirement for creosote oil was 135 kg per one cubic meter of impregnated sapwood. When using other vegetable oils, it is known that the amount has to be multiple compared with creosote oil in order to achieve a preservation effect of the same level against decay. It is possible to obtain such large amounts of oil within wood only by means of pressure impregnation.
As there must be large amounts of wood preservatives containing vegetable oils within wood, a problem has been that the wood preservative tends to bleed onto the surface of wood after pressure impregnation. This problem is especially caused by the infiltration of wood preservatives i.e. a considerably larger amount of wood preservative remains on the surface layers of wood than deeper in the wood. This phenomenon has been especially emphasised when impregnating green wood or wood having poor permeability for some other reason. The poorer the permeability of wood, the greater the infiltration is and the more easily the wood preservative bleeds onto the surface of wood. The infiltration can be very strong i.e. there can be a multiple amount of wood preservative on the surface layer of wood compared with the inner parts of wood. Bleeding occurs also during the use of wood, especially when the surface of wood heats e.g. by the effect of the heat of the sun. The wood preservative bleed onto the surface makes the outer surfaces of wood sticky. The sticky surface of wood is unpleasant as such and becomes dirty easily and its surface treatment e.g. by painting does not turn out well. In Finnish patent FI 114295 B, this problem was solved by drying wood to the moisture content of less than 10% before impregnating with a wood preservative containing vegetable oil and by heating the wood at the end of treatment to a temperature of over 150° C. for polymerising and drying the oils contained by the wood preservative. The amount of oil-bearing wood preservative in the wood treated in this way is however small (only about 30 liters per one cubic meter of treated wood), because in this method, the wood is impregnated without pressure. It is thus evident that, because of the small amount of wood preservative, there has been almost no bleeding problem in the case of wood treated by impregnation. In practice, it has been noticed that, e.g. in the case of pine, the bleeding of oil-bearing wood preservative increases considerably when the average content of wood preservative within wood increases over 100 kg per one cubic meter of impregnated wood. In the surface layer of pine containing this amount of wood preservative, there can be even over 200 kg/m3 of wood preservative. It has been tried to solve the problem by the polymerisation/drying of wood preservative using high temperatures of over 130° C. and/or by adding catalysts and drying agents in the wood preservative. With these, it has been possible to decrease the bleeding because more solid wood preservative blocks up the cellular structure of the surface layers of wood so that the wood preservative expanding when heated cannot easily exit the wood. With the method, one has not been able to totally prevent the bleeding of the wood preservative and disadvantages caused by it. Furthermore, the use of the method impedes the actual impregnation process, because it hinders the penetration of wood preservative within the wood and, thus, the wood preservative is not evenly spread in the wood cells. It is also probable that the polymerisation weakens the ability of the preservative to prevent moulding and decay, because then the biocide components of oil are not able to affect the organisms contributing to moulding and decay. In addition, e.g. in the case of wood poles impregnated in this way, dryer agents and the polymerisation of oil prevent the natural run-off of the oil-bearing wood preservative at the base of the pole and on ground level where the risk of decay is the greatest.
The object of the invention is to provide a method by means of which above-mentioned problems related to known methods are eliminated. The object of the invention is especially to introduce a method by means of which pressure impregnated wood can be manufactured with a wood preservative containing vegetable oil so that the preservative does not bleed onto the surface of wood after impregnation, even though no agents causing polymerisation or drying agents were added to the preservative and even though large amounts of wood preservative containing vegetable oil were impregnated within the wood. Furthermore, the object of the invention is to introduce a method by means of which also green wood (moisture content of wood over 28%) and poorly impregnatable wood, e.g. pine heartwood and spruce, can be pressure impregnated with vegetable oil based wood preservatives so that the wood preservative does not bleed onto the surface of wood after pressure impregnation. The object of the invention is further to introduce wood pressure impregnated with a wood preservative containing vegetable oil the wood preservative of which does not bleed onto the surface of wood during the use of wood, even though a large amount of the wood preservative containing vegetable oil has been impregnated in the wood.